IR1168-with-RC-filter(1)

COMPANYCONFIDENTIAL1IR1168–AddRCfiltertoVDHelenDingSystemApplicationsJuly/6/2009COMPANYCONFIDENTIAL2SmartRectifierTMinLLCHalf-BridgeLOADSR1SR2Cdc12LrC1C2RtnVinVD25VS26VS13VCC2VD14GND7GATE28GATE11IR1168CoutRg2Rg1M1M2ThetypicalapplicationcircuitofIR1168inLLCapplicationCOMPANYCONFIDENTIAL3DCMoperationandVDringingn:1:1VSLs2Ls1LrCrLmCOUTM2LOADVinM1HOLOM3M4Cp1Cp2VD1VD2IS2IS1VSVCrILrILmHOLOIS2IS1Belowresonant(FswFr1)ZVSswitchingOutputCurrentDCMVD2TDCTDCMostapplicationsweredesignedtoworkunderbelowresonantmode(FswFr)tohavewiderregulationrange.TypicallythesecondarycurrentisinDCMmodeunder“BelowResonantZVS”.DuringthetimeTDC,theprimarymagneticcurrentequalstotheresonantcurrent,thereisnoenergydeliveryfromprimarytosecondary.Thecurrentintransformersecondaryiszero.TheoutputloadcurrentissuppliedbytheCOUTcapacitor.Thetransformersecondaryinductorwillresonantwiththeparasiticcapacitors(suchastheSRMOSFETCosscapacitorandtransformerwindingcapacitor)withintheTDCtime.ThatcausesafreeringingappearsontheVDofSRMOSFET.COMPANYCONFIDENTIAL4FalseTriggeringunderDCMoperationInanideacase,thefreeresonantamplitudeequalstotheoutputvoltagelevel.ItcanbehigherthanVoutifithasasmallamountofreversecurrentintheSRMOSFET.Thiscouldhappenduringthereverserecoverytimeofthebodydiode.ThusithastherisktofalsetriggerIR1168(2).AsIR1168(2)willturnonMOTtimeoncetriggered,itwillforcetheoutputcapacitordischargethroughSRMOSFET(reversecurrent)andcausespikeonMOSFETattheFETturn-off.COMPANYCONFIDENTIAL5FalseTriggeringunderDCMoperationThiskindoffalsetriggeringcouldnotbeavoidedbytheblankingtimeofIR1168.Thisisbecausethe15usblankingtimeisalwaysresetbyVDbeforethefalsetriggering(VDvoltageisaboveVth3).TheMOTprotectioncircuit(inIR11682)alsocouldn’tsolvethisissueifitseesanormalpulsebeforethefalsetriggeringpulse.Thenormalcyclewillresetthelatchofprotectionandenablethenextgateoutput–thatmeansthefalsetriggeringisenabledwithnoprotection.*TheMOTprotectioncircuitinIR11682onlycouldpreventthenextfalsetriggeringormultifalsetriggering.COMPANYCONFIDENTIAL6¾AnymethodtoreducetheamplitudeofDCMringing:™AddRCsnubbertoMOSFETtochangeringingfrequencyanddamptheamplitude.Effectivebutwillcauseadditionalpowerlosses.™AddsmallSchottkydiodeinparallelwithSRMOSFETtoreducereverserecoverycurrent.Losslessbutaddcost(mayneedbiggerdiodedependsontheresult).™AddRCfiltertoVDpin.Flexibletochangethefiltertimeconstant,butwilladdbothturn-onandturn-offdelay.¾TriggerSRoutputonlywhenVDnegativepulseiswiderthan150ns.™Equivalenttoadd150nsturn-ondelay.Won’taffecttheturn-offdelay.Affectefficiencyalittlebit.™The150nsisonlygoodtofilter3MHzringing.SolutionsforDCMfalsetriggeringCOMPANYCONFIDENTIAL7TheschematicofIR1168withRCdelaytoVDGATE11VCC2VS13VD14GATE28GND7VS26VD25IR1168VinRtnRCfiltervalue:R–below2KohmC–limitedbypowerlossofCV2Fs(thepowerwillbedissipatedintheR),suggestedtobebelow100pFCOMPANYCONFIDENTIALTime232us234us236us238us240us231us241usV(Vdrain1)V(VD1)10V20V-2VSELI(M3:3)V(GATE1)V(VD1)-1001020308ThesimulationresultofIR1168withRCdelaySimulationwith1.5Kohm+22pFRCfilter.Inthebottomplot:Greenwaveform:MOSFETDrainvoltagebeforeRCfilter.Bluewaveform:VD1voltageaftertheRCfilter.TheresultofRCfilterisobviousforhighfrequencyringing.Itcouldeffectivelyreducetheamplitudeofringingsothatavoidfalsetriggering.TheRCfilterhaslesseffectforsmoothsignalsotheturn-offphaseofIR1168doesnothavetoomuchdelay.12COMPANYCONFIDENTIAL9ThesimulationresultofIR1168with1.5K+22pFZoominofarea1.Theturn-ondelaythatcausedbytheRCfilteris90ns,around3*RC(timeconstant).Zoominofarea2.Theturn-offdelaythatcausedbytheRCfilteris30ns(doesn’tshowinthisvoltagescaleinthisplot),itisaroundtheRCtimeconstant.Time232.6us232.8us233.0us233.2us233.4us233.6us233.8us234.0usV(Vdrain1)V(VD1)0V5.0V8.0VSELTime237.8us238.0us238.2us238.4us238.6us238.8us239.0us239.2usV(Vdrain1)V(VD1)0V5.0V8.0VSELavoidfalsetriggeringavoidfalsetriggeringadd90nsturn-ondelayadd30nsturn-offdelayCOMPANYCONFIDENTIAL10ƒAddRCfiltertoVDpincaneffectivelyfiltertheringing.Thetimeconstantshouldbeintensofnsrange.Theadditionalturn-ondelayisaround3RC,theadditionalturn-offdelayisaboutoneRCtimeconstant.ƒItisflexibletochoosedifferentRCfilterfordifferentsystem.ƒTheturn-ondelayaffectefficiencyalittlebitandshouldbenegligibleaslongasthetimeconstantisbelow50ns.ƒTheturn-offdelaycouldcausereversecurrentwithDrectFETunderCCMmode.Theearlyturn-offeffectwiththrough-holepackagedMOSFETcouldhelpeasingthedelay.Soitwon’tbeanissueiftheRCtimeconstantisbelow50nswithTO220MOSFET.SummaryofRCfiltertoVDpin